Commitment of B lymphocytes to a plasma cell fate is associated with Blimp-1 expression in vivo.

B lymphocyte-induced maturation protein-1 (Blimp-1) is a transcriptional repressor that is sufficient to trigger terminal differentiation in the B cell lymphoma BCL-1. In this study, we have determined the expression pattern of Blimp-1 in vivo in primary and secondary lymphoid organs of humans and immunized mice. Blimp-1 is expressed in plasma cells derived from either a T-independent or T-dependent response in plasma cells that have undergone isotype switching and those resulting from secondary immunization. Blimp-1 is also present in long-lived plasma cells residing in the bone marrow. However, Blimp-1 was not detected in memory B cells. This expression pattern provides further evidence of a critical role for Blimp-1 in plasma cell development, supporting earlier studies in cultured lines. Significantly, Blimp-1 was also found in a fraction (4-15%) of germinal center B cells in murine spleen and human tonsils. Blimp-1 expression in the germinal center is associated with an interesting subset of cells with a phenotype intermediate between germinal center B cells and plasma cells. In the mouse, Blimp-1(+) germinal center B cells peak at day 12 postimmunization and disappear soon thereafter. They are not apoptotic, some are proliferating, they express germinal center markers peanut agglutinin or CD10 but not Bcl-6, and most express CD138 (syndecan-1), IRF4, and cytoplasmic Ig. Together, these data support a model in which B cell fate decisions occur within the germinal center and Blimp-1 expression is critical for commitment to a plasma cell, rather than a memory cell, fate.

Transcriptional repression by blimp-1 (PRDI-BF1) involves recruitment of histone deacetylase.

B-lymphocyte-induced maturation protein (Blimp-1) is a transcriptional repressor that is considered to be a master regulator of terminal B-cell development because it is sufficient to trigger differentiation in the BCL(1)-cell model. Transcription of the c-myc gene is repressed by Blimp-1 during B-cell differentiation. In this study, we have explored the mechanism by which Blimp-1 represses transcription by using Gal4-fusion protein assays and assays in which Blimp-1 represses the natural c-myc promoter. The results show that Blimp-1 represses the c-myc promoter by an active mechanism that is independent of the adjacently bound activator YY1. Blimp-1 contains two regions that independently associate with histone deacetylase (HDAC) and endogenous Blimp-1 in nuclear extracts binds in vitro to the c-myc Blimp-1 site in a complex containing HDAC. The functional importance of recruiting HDAC for Blimp-1-dependent repression of c-myc transcription is supported by two experiments. First, the HDAC inhibitor tricostatin A inhibits Blimp-1-dependent repression in cotransfection assays. Second, a chromatin immunoprecipitation assay shows that expression of Blimp-1 causes deacetylation of histone H3 associated with the c-myc promoter, and this deacetylation depends on the Blimp-1 binding site in the c-myc promoter.

BLIMP-1: trigger for differentiation of myeloid lineage.

B lymphocyte-induced maturation protein-1 (BLIMP-1 or PRDI-BF1) is induced when bone marrow-derived progenitors differentiate in response to macrophage-colony stimulating factor (M-CSF) and is present in peripheral blood monocytes and granulocytes. BLIMP-1 is also induced during differentiation of U937 and HL-60 cells into macrophages or granulocytes. Induction of BLIMP-1 mRNA during macrophage differentiation of U937 and HL-60 shows a biphasic pattern. Overexpression of BLIMP-1 is sufficient to initiate macrophage differentiation of U937 cells whereas blocking endogenous BLIMP-1 inhibits differentiation. One target of BLIMP-1-dependent transcriptional repression in U937 cells is c-myc, providing an explanation for cessation of cell division. Thus BLIMP-1 is a key regulator of terminal differentiation in two separate hematopoietic lineages: myeloid cells and B lymphocytes.

Mother-to-child transmission of human T-cell-leukemia/lymphoma virus type I: implication of high antiviral antibody titer and high proviral load in carrier mothers.

In order to gain new insights into the risk factors influencing human-T-cell-leukemia/lymphoma-virus-type-I (HTLV-I) mother-to-child transmission, a retrospective study of HTLV-I infection among children born to HTLV-I-seropositive women was carried out in a highly HTLV-I-endemic population of African origin living in French Guyana. The study covered 81 HTLV-I-seropositive mothers and their 216 children aged between 18 months old and 12 years old. All plasma samples were tested for the presence of HTLV-I antibodies by ELISA, immunofluorescence assay and Western blot. HTLV-I provirus was detected, in the DNA extracted from peripheral-blood mononuclear cells, by polymerase chain reaction (PCR) using primers specific for 3 different HTLV-I genomic regions (LTR, gag and pX) and quantified by a competitive PCR assay. Out of the 216 children, 21 were found to be HTLV-I-seropositive, giving a crude HTLV-I transmission rate of 9.7%, while among the 180 breast-fed children 10.6% were HTLV-I-seropositive. Perfect concordance between serological and PCR results was observed, and none of the 195 HTLV-I-negative children was found HTLV-I-positive by PCR. In conditional (by family) logistic-regression models, HTLV-I seropositivity in children was associated with an elevated maternal anti-HTLV-I-antibody titer (OR 2.2, p = 0.0013), a high maternal HTLV-I proviral load (OR 2.6, p = 0.033) and child's gender, girls being more frequently HTLV-I-infected than boys: OR 3.6, p = 0.0077 in the model including maternal anti-HTLV-I-antibody titer and OR 4.1, p = 0.002 in the model including the maternal HTLV-I proviral load.

Evidence that immunoglobulin VH-DJ recombination does not require germ line transcription of the recombining variable gene segment.

The importance of V(D)J recombination for generating diversity in the immune system is well established, but the mechanisms which regulate V(D)J recombination are still poorly understood. Although transcription of unrearranged (germ line) immunoglobulin and T-cell receptor gene segments often precedes V(D)J recombination and has been implicated in its control, the actual role of germ line transcripts in V(D)J recombination is not known. We used a sensitive reverse transcription-PCR assay to study immunoglobulin VH germ line transcripts in proB lines from RAG-deficient mice. All 10 VH families analyzed were germ line transcribed, and germ line transcription was found in all of the cell lines examined, indicating that active chromatin was present in the VH region. However, not all VH families were germ line transcribed in every cell line, and there was a surprising lack of uniformity in the number and family distribution of germ line VH transcripts in individual lines. When V(D)J recombination was activated by restoration of RAG activity, recombinational activity of endogenous VH genes for which germ line transcription was observed could be compared with those of genes for which it was not observed. This analysis revealed multiple examples of endogenous VH gene segments which were rearranged in cells where their germ line transcription was not detectable prior to RAG expression. Thus, our data provide strong support for the idea that V-(D)J recombination does not require germ line transcription of the recombining variable gene segment.

Human herpes virus 8 (Kaposi's sarcoma herpes virus) and malignant lymphoproliferations in France: a molecular study of 250 cases including two AIDS-associated body cavity based lymphomas.

The new human herpes virus 8 (HHV8) was recently detected in cases of body cavity based lymphoma (BCBL), a rare B cell lymphoma, mostly AIDS-associated. We investigated for HHV8 DNA sequences a series of 250 B or T cell lymphoproliferative malignancies, as seen in France, including 126 leukemias and 124 lymphomas (232 non-AIDS-associated and 18 AIDS-associated tumors). HHV8 sequences were detected in only three patients. The first two were homosexual males, HIV-infected since 1985 who suffered from a BCBL initially characterized in one case by a pleural lymphomatous effusion and a peritoneal one in the other case. A high level of HHV8 copies was detected in the tumoral cells of these two BCBL. In contrast, in the third positive patient who had an AIDS-associated immunoblastic lymphoma, the HHV8 sequences level was quite low. In the two BCBL patients, the HHV8-infected clonal B cells had a large immunoblastic feature with an indeterminate phenotype and were also infected by Epstein-Barr virus. In one BCBL case, a semiquantitative PCR analysis revealed that the HHV8 sequences were much more abundant in the effusion tumor cells than in the cutaneous Kaposi's biopsy while no HHV8 sequence was detectable in the peripheral blood lymphocytes. This study reports HHV8-associated BCBL in European AIDS patients and confirms that HHV8 is present at a high copy number in the tumoral B cells of this malignancy. Furthermore, HHV8 does not seem to play a pathogenic role in any of the other T or B malignant lymphoid neoplasias studied so far. This study also stresses the necessity for quantification studies in interpretation of a positive PCR analysis for HHV8 sequences, especially in patients at risk for HIV infection or Kaposi's sarcoma.